This invention relates to a control device for use in an internal combustion engine and a continuous variable transmission, wherein the control device is connected to the continuous variable transmission arranged in a power transmission method between driving wheels and the internal combustion engine loaded on a vehicle. More particularly, this invention relates to a control device of this type in which power of an internal combustion engine can be adjusted by means of controlling an intake air flow adjusting unit and which gear ratio of the continuous variable transmission can be changed at a variable speed suitable for driving conditions of a vehicle.
Generally, engine power (hereinafter referred as power) of an internal combustion engine (hereinafter referred as an engine) mounted on a vehicle is mechanically controlled by a throttle device. The throttle device is coupled to an accelerator pedal and a throttle lever (both of which are referred as accelerator hereinafter) through an accelerator cable.
However, when the accelerator and the throttle device directly cooperate with each other, excessive power may be generated due to careless driving and lack of skill of a driver. The excessive power will cause many troubles: a car may slide when a driver steps the accelerator; slip on the icy ground; skid at a sudden acceleration.
With respect to the above-mentioned problems, many method have been proposed for the control device, such as a dual throttle valve method and a traction control (power control) method utilizing so called drive-by-wire method. In the dual throttle valve method, a main throttle valve and a sub throttle valve are arranged in the throttle device and the sub throttle valve is electronically controlled. The drive-by-wire method is one such that the accelerator cable is not disposed between the accelerator and the throttle valve. An accelerator opening (throttle opening) is detected by using a sensor such as a potentiometer, while the throttle valve is operated by a stepping motor or the like.
With the traction control method, the internal combustion engine and the continuous variable transmission are generally controlled by using an engine control unit (ECU). The ECU calculates an optimum opening for the main throttle valve and the sub throttle valve in accordance with the data representative of the rotation speed of the front and rear driving wheels as well as a step amount for the accelerator pedal. In this way, a driving shaft torque of the wheels is controlled so as not to cause the undesirable skid or the like.
An information regarding to the required power of the engine is properly determined in accordance with the data representative of, for example, the accelerator opening. As mentioned above, the ECU calculates the required power of the engine when using the traction control method for controlling the internal combustion engine and the continuous variable transmission. The sub throttle valve and the main throttle valve are controlled so as to obtain the calculated required power. In this event, it is preferable that the calculation is carried out with respect to a real torque of the current condition. More specifically, an over-control and a bad responsibility of the control device can be prevented by calculating a deviation between a required torque and the real torque and by carrying out a real time control to eliminate the deviation.
The real torque of the engine can be detected under a bench test using a chassis dynamometer. However, it is difficult to mount such a chassis dynamometer on a vehicle due to its weight, size, and costs. In addition, power (energy) loss is inevitable when using the chassis dynamometer for detecting the real torque of the engine.
On the contrary, a power torque of an engine is transmitted to a driving shaft via a transmission in the power transmission method for the engine. For example, a continuous variable transmission (CVT) is applied to a vehicle as such a transmission. The CVT can adjust a gear ratio in response to a signal indicative of hydraulic pressure which is applied to a hydraulic actuator.
In the continuous variable transmission of the type described, a gear speed is calculated so as to eliminate a gear ratio deviation between a desired gear ratio, calculated in accordance with a driving condition, and a real gear ratio. Accordingly, the hydraulic actuator Of the CVT can be controlled in order to obtain an optimum variable speed.
The above-mentioned conventional continuous variable transmission for a vehicle controls the hydraulic actuator so as to correct the real gear ratio into the desired gear ratio. However, a high/low level of the torque transmitted on control in not considered.
Accordingly, when the gear ratio of the CVT increases or decreases at a relatively low speed, the torque on the driving shaft is smoothly changed. Further, when the gear ratio deviation between the desired gear speed and the real gear speed is relatively large, the gear speed rapidly increases or decreases. However, the CVT consumes a gear operation torque and a moment of inertia of pulleys is relatively high. This may cause an opposed reaction against the gear acceleration with rough gear operation. In addition, excessive torque of the engine may result in slide of a steel belt. Such a "shock" on transmission is reduced merely by restricting the transmission rate, which results in low transmission performance of the continuous variable transmission.